Unfortunately however much the Cabin Crew are asked in memos etc not to do it they will....I suppose the extensive few weeks training they get doesn't cover this...only way is to get them to assist in clearing the sink blockage....as a previous post said, believe it was QF that used to make the crew stay at the end of a flight and help clear any blocked sinks.....soon sorted that problem out....yes I know its not the ideal situation having to put the waste down a toilet pan but that is how the aircraft are.....if you don't like it then get another job or join CX as they appear to have a proer disposal system in the galley......crap down the galley sinks doesn't just cause the inconvinience of a blocked sink it also causes problems with the aerials etc as has been mentioned earlier....personally, if I am on a turnround and the sink is blocked, it is the last thing I attend to...it is the least important, if the cabin crew complain then I have great pleasure in pointing out why it is blocked......if I get time I will try and clear it...